As for the high-voltage insulated gate type semiconductor device, an IGBT (Insulated Gate Bipolar Transistor) has been widely used. FIG. 1 shows a structure diagram of the IGBT.
As shown in FIG. 1, an IGBT structure includes trenches 2 and 3 selectively formed in a front surface of a low-concentration N type layer (N base layer 1) with large intervals and narrow intervals provided alternately, gate insulating films 4 and 5 formed on surfaces of the trenches 2 and 3, respectively, polysilicon gate electrodes (control electrodes) 6 and 7 formed inside the gate insulating films 4 and 5, respectively, a P base layer (P well layer) 8 selectively formed between the adjacent trenches sharing the narrow interval, a high-concentration N source layer 9 selectively formed on a front surface of the P base layer 8, and an emitter electrode 10 connected to each of the P base layer 8 and the N source layer 9. A MOS transistor structure is formed in surface portions of the N source layer 9, the P base layer 8, and the N base layer 1, and a P type layer (a P type layer 11 having an unfixed potential) is formed between the adjacent trenches sharing the large interval to have the same depth as the trench and so as not to be connected to the emitter electrode 10 or so as to be connected to the emitter electrode 10 through a high resistor. Furthermore, the IGBT structure includes an N buffer layer 12 uniformly formed on a back surface of the N base layer 1 so as to have a higher impurity concentration than the N base layer 1, a high-concentration P type layer (P emitter layer 13) uniformly formed on a front surface of the N buffer layer 12, and a collector electrode 14 uniformly formed on a front surface of the P emitter layer 13.
As for the IGBT having the trench structure, various kinds of proposals and studies have been made as disclosed in Patent Document 1 and Non-patent Documents 1 to 9.